If the major issue is demon-bombing with light cruisers, then I wouldn't have a problem restricting demon ships to 8+ hull points. It would basically put a 40pt virtual tax on a demon ship compared to a Heretic or an Unbeliever, but it would also make sitting phased in the middle of someone's fleet a more expensive & wasteful proposition.
If the Heretic seems a little unimaginative as a CL, it could get swapped for the Siluria, whose lore makes it an ancient light cruiser design. Of course, that gets Chaos an even cheaper light cruiser, but I could see letting people use their Heretics as Silurias, and it would keep the reserves bleed going from the Havoc frigate.
Under the updated fleet list costs, 3 Dauntless and an Emperor clocks in at 695pts, 3 Unbelievers and a Despoiler would cost 765pts, 3 Unbelievers and a Desecrator would be 740pts, and a Desolator/Relictor + 3 comes in at 675pts. Of the three builds that could squeak in under the 750pt cap, none of them have the threat projection that the Emperor could bring to the table (ok, maybe the Relictor, but in a different way), so I'm not sure the option to go top-heavy is, comparatively at least, that much of an issue (not that I'd want to face 3CLs + an Emperor either, and a raiding fleet player would probably love it less). Of course, swapping out the Heretic for the Siluria would make a Despoiler bomb raid fleet viable, so be careful what you wish for. These kinda of fleet comps make me really want Horizon's mandatory escort-to-capital ship ratio, even if its just 1:1 for cruisers and 2:1 for battleships, since that would kill the "battleship in a raid" option.
